A manipulator has a stationary base on which a carousel is rotatable about a vertical axis by means of a motor mounted on the base. A main support arm is rotatable about a horizontal axis perpendicularly intersecting the vertical axis by means of a motor mounted on the carousel. A counterweight is provided for counterbalancing the offcenter weight of the main arm and the structure carried by it.
The upper end of the main arm carries an outrigger arm rotatable on the main arm about a horizontal axis by means of a motor carried on the main arm. The outer end of this outrigger arm carries a so-called mechanical hand comprising a head rotatable about a head axis perpendicularly intersecting the outrigger-arm axis and carrying in turn a tool holder rotatable on the head about a holder axis perpendicularly intersecting the axis. This holder in turn can rotate a tool about a holder axis perpendicular to the head axis. Drives are provided at the rear end of the outrigger arm for rotation about the various axes.
Such an arrangement is also described in my U.S. Pat. No. 4,548,097. This arrangement has a main support defining a main longitudinal axis, a housing rotatable on the main support about the main longitudinal axis, a head defining a head longitudinal axis and rotatable on the head housing about an outer transverse axis crossing the main longitudinal axis, and a holder rotatable on the head about the head longitudinal axis. Respective housing, head, and holder stepdown drive transmissions have outputs connected directly to the housing, head, and holder and inputs and have stepdown transmission ratios of at least 70:1. Respective head and holder drive trains include respective head and holder right-angle drives having input shafts coaxial with the main longitudinal axis and coaxial output shafts defining an inner transverse axis generally parallel to the outer transverse axis but offset along the main longitudinl axis therefrom toward the support, respective head and holder output members connected to the respective transmission inputs and both rotatable about the outer transverse axis, and respective head and holder connecting elements between the respective output shafts and output members for joint rotation of each output member with the respective output shaft.
A serious problem in such arrangements is accurately driving the joints at least at the so-called main axes, that is those axes relatively close to the stationary base, as any slip or play at these axes is magnified at the tool end. This problem is particularly troublesome when the tool must be moved back and forth, as a transmission that is slipfree in one direction normally slips slightly as it reverses and all the play in it is taken up in the opposite direction.
My earlier U.S. Pat. No. 4,399,718 describes a pivot drive for interconnecting a pair of members that are relatively pivotal about a pivot axis. This drive has a torsion rod extending along a rod axis and rotatable by an appropriate drive relative to one of the members. This rod has outer ends carrying input gears of respective constant-mesh gear trains having output gears at least one of which is limitedly rotatable about the pivot axis on the other member. This one output gear can be limitedly rotated and locked in place in the other member so as torsionally to prestress the rod, thereby in theory eliminating play from the drive by bringing the gears of one of the gear trains into snug contact in one direction and the gears of the other gear train into snug contact in the opposite direction.
This arrangement is unfortunately only effective when coaxial, planetary-gear stepdown transmissions are used. Unfortunately such transmissions are not efficient at large stepdown ratios. In addition it is very expensive and not altogether possible to make such an arrangement equally tight for rotation in either direction, so that some difference in slip is encountered on direction change.
Another solution has been proposed in German Pat. No. 2,226,407 filed 31 May 1972 by Hans Richter. This arrangement uses noncoaxial toothed-belt transmissions. Such construction does indeed permit high stepdown ratios to be achieved with low losses, but these transmissions all have some slip or play. Admittedly the play of the upstream transmissions is reduced with each subsequent stepdown, but that of the furthest downstream transmission is transmitted directly to the output arm.